A quantum well typically is made by forming a narrow bandgap semiconductor layer between two wide bandgap semiconductor layers. The narrow bandgap semiconductor layer is referred to as the quantum well layer and the wide bandgap semiconductor layers are referred to as barrier layers. Multiple quantum wells may be formed from an alternating series of narrow bandgap semiconductor layers and wide bandgap semiconductor layers.
Quantum well structures have been incorporated into a wide range of optoelectronic devices, including EAMs (Electro-Absorption Modulators), EMLs (Electro-absorption Modulated Lasers), detectors, wave-guides, inline optical amplifiers, as well as integrated structures that use a combination of these devices.
Polarization independence is a desirable feature for many types of optoelectronic devices. The quantum confinement in semiconductor quantum well devices, however, lifts the degeneracy of the heavy-hole and light-hole valence bands. In unstrained semiconductor material, the heavy-hole ground state is lower in energy than the corresponding light-hole ground state. The resulting differences in the bound state energies for light-holes and heavy holes causes the light holes and the heavy holes to couple differently to electromagnetic waves. For example, TE (transverse electric) mode light couples conduction band electrons with the heavy-hole valence band, whereas TM (transverse magnetic) mode light couples conduction band electrons with the light-hole valence band. Depending on the semiconductor materials and the type of strain, one of the light-hole or heavy-hole valence bands is shifted to a higher energy level while the other valence band is shifted to a lower energy level. Therefore, in many semiconductor quantum well devices the bandgap energy, which determines the optoelectronic properties of the device, is a function of the polarization of light traveling through a quantum well structure.
Different types of polarization-independent semiconductor quantum well devices have been proposed. In one approach, a slight tensile strain is introduced in the quantum wells to move the transition energies for light holes and heavy holes to the same energy level. In this way, the wave-guide dispersion for TE and TM polarizations may be made small enough to achieve polarization independence. Another approach has used a combination of two groups of quantum wells that are interspersed in a single active region and are suitably configured such that one group of quantum wells provides TE mode optical coupling and the second set of quantum wells provides TM mode optical coupling. In this approach, the waveguide dispersion for TE and TM polarizations is substantially reduced by configuring the two quantum well groups (one set for TE and a second set for TM) so that the level of TE mode optical coupling is approximately the same as the level of TM mode optical coupling.
In each of the above-described approaches for achieving polarization-independent operation, all of the electron-hole transitions occur within the quantum wells. Because of the conservation of the oscillator strength/matrix element, the available oscillator strength in the slightly tensile strained quantum well active structure is split equally between TM-mode coupling and TE-mode coupling. Hence the level of optical absorption available for a given polarization is roughly one-half of what it would be if the device were designed for a single type of polarization. For the case with two separate groups of quantum wells (one set for TE and the other set for TM polarization), a relatively thicker (e.g., twice as thick) quantum well active region is needed to achieve the same level of coupling for each polarization mode as a device designed for a single type of polarization, but this is not always feasible. In this approach, the waveguide effectively has been diluted by including quantum wells for both polarizations. Complications such as carrier transport problems as well as output coupling problems due to the stronger optical confinement also may arise in this approach.